In optical reading type disk shape recording media, such as an optical disk and magneto-optical disk, various standards of products are on the market, including one that is read only and one on which recorded information can be overwritten. Such recording media are manufactured, in many cases, by laminating a pair of substrates with the adhesive layer in between, in order to protect the recording face formed on the substrate, or to implement high density recording by making the recording face multilayered.
Such a lamination type disk is manufactured, for example, as follows. Two polycarbonate substrates are injection-molded, and metal film (recording film) for reflecting a laser is formed by sputtering in a sputtering chamber. And an ultra violet-hardening type adhesive is coated on the bonding faces of the two substrates, and adhesive is spread by spin coating. Spin coating involves coating adhesive around the center of the substrate, and then forming a thin film of adhesive (adhesive layer) on the substrate by spinning the substrate at high speed.
The pair of substrates on which the adhesive layer is formed are inserted into a vacuum chamber, and the adhesive layers are laminated to each other in the vacuum. The substrates laminated to each other are removed from the vacuum chamber into the atmosphere, and ultraviolet is irradiated onto the entire substrates to harden the adhesive. By this the two substrates are firmly bonded, and the disk completes.
The substrates laminated together, as above, must be flat, without warp or distortion, so that a spot is stably formed when a laser to be used for reading and writing of information is irradiated onto the disk. Therefore in such an optical type disk, it is desirable that the film thickness of the adhesive layer of laminating is as uniform as possible. However, in the case of the above mentioned spin coating, where adhesive is spread by centrifugal force, the thickness at the outer circumference becomes thicker than that at the inner circumference (e.g. about 10 μm), and it is difficult to make film thickness uniform on the entire substrate.
To handle this problem, Japanese Patent Application Laid-Open No. 2001-209980 proposes a technology for making film thickness uniform on the entire substrate by laminating two substrates and hardening the adhesive of the inner circumference side first by irradiating ultraviolet on only the inner circumference side while rotating, then spreading the adhesive of the outer circumference side, and irradiating ultraviolet again on the entire surface for hardening the adhesive. Japanese Patent Application Laid-Open No. 2003-340359 proposes a technology for making film thickness uniform on the entire substrate by coating adhesive on the substrate, then irradiating ultraviolet on only the inner circumference side while rotating so as to partially increase viscosity, and spreading adhesive of the outer circumference side.
Also Japanese Patent Application Laid-Open No. 2002-63737 and No. 2004-39050 disclose prior arts, where a convex section is formed when adhesive is formed using centrifugal force, so that contact of the surfaces of the disks is prevented when completed disks are superimposed, although this is difference from a technology for making film thickness uniform.
However the technology disclosed in Japanese Patent Application Laid-Open No. 2001-209980 is for making the adhesive layer uniform by laminating the substrates first, then controlling irradiation of ultraviolet and the rotation of the substrate, so if the thickness of the adhesive on the substrate before lamination is not uniform, warp or distortion is generated in the substrate during lamination, and correction in later processing becomes difficult. Although the inner circumference side is hardened first by irradiating ultraviolet, it takes time to set the optimum values of the irradiation range, irradiation time and rotation frequency so that no difference is generated between the inner circumference portion which has already been hardened and of which thickness has been determined, and the unhardened portion. Actually, as Embodiment 2 of Japanese Patent Application Laid-Open No. 2001-209980 shows, optimum values must be adjusted while measuring the film thickness for each lot using a film thickness measuring device, which is an involved procedure.
Japanese Patent Application Laid-Open No. 2003-340359 is not for coating adhesive for laminating together substrates but for forming the coating layer on the surface of the substrates, so hardening is progressed by irradiating ultraviolet onto the inner circumference side, and therefore this cannot be applied to a technology for laminating together substrates. Also just like Japanese Patent Application Laid-Open No. 2001-209980, it takes time to set such optimum values as irradiation range, irradiation time and rotation frequency. In Japanese Patent Application Laid-Open No. 2002-63737 and No. 2004-39050, a convex portion remains on the surface of the disk, which is incompatible with uniform coating.